1. Field of the Invention
This invention relates to a method and apparatus for improving aircraft safety. More specifically, it relates to an apparatus and method for preventing combustion in aircraft fuel tanks and cargo spaces by supplying inert gas at flow rates determined by a pressure scheduling valve.
2. Description of the Related Art
Military aircraft have used On-board Inert Gas Generating Systems (OBIGGS) for some years to protect against fuel tank explosions due to undesired phenomena, such as penetration from small arms fire. Military aircraft are not the only aircraft that would benefit from OBIGGS. For example, investigations into the cause of recent air disasters have concluded that unknown sources may be responsible for fuel tank ignition and explosion. Subsequently, OBIGGS has been evaluated as a way to protect commercial aircraft against such fuel tank explosions started by unknown ignition sources.
OBIGGS protects against fuel tank explosions by replacing the potentially explosive fuel/air mixture above the fuel in the tanks (the ullage) with an inert gas (usually nitrogenxe2x80x94N2). The nitrogen is generated by separating oxygen from local, ambient air and pumping the inert product into the tanks.
In previous applications, OBIGGS has proved relatively unreliable, heavy, and costly for both initial acquisition and operation in aircraft. Furthermore, military aircraft systems often have the strict requirements that derive from military flight profiles that include high-rate descent from high-altitude flight. Applications of OBIGGS to commercial aircraft would benefit by considering the fact that a typical flight profile for the commercial application has a less demanding requirement for the system because commercial aircraft do not operate under the same conditions of military aircraft.
Thus, there is a need for a gas generation and inerting system that minimizes the quantity and complexity of sub-components (particularly moving parts). The system should also consider a typical commercial aircraft flight profile and take advantage of the reduced descent rates (compared to military aircraft).
The present invention provides an apparatus and method for reducing the possibility of combustion in aircraft fuel tanks by replacing air in the ullage of the fuel tank with an inert gas that has been separated from the engine bleed air. The apparatus includes an air separation module, a filter, a heat exchanger, and a pressure scheduling valve. In one preferred embodiment, a single housing contains at least the air separation module, filter, heat exchanger, and pressure scheduling valve. The air separation module can be mounted in a center-section of the housing, may be a canister-type design, may include a replaceable cartridge, and may be a hollow-fibre permeable membrane configuration.
The method of the present invention includes displacing the atmosphere in the ullage of a fuel tank with a non-combustible gas. In one preferred embodiment, the pressure scheduling valve taps into the pressure of the air supplied to the air separation module. Based on this pressure the pressure scheduling valve adjusts a variable orifice so that the system introduces a non-combustible gas to the ullage at a variable flow rate and variable purity level. The flow rate and purity level range from a high-purity non-combustible gas at a low flow rate when the supplied air pressure is relatively high, to a lower-purity noncombustible gas at a higher flow rate when the supply pressure is relatively low. In an embodiment of the invention, the pressure scheduling valve taps into the pressure of the nitrogen-enriched air discharged from the air separation module and bases the variable orifice adjustments on pressure of the nitrogen-enriched air. In one embodiment of the invention the non-combustible gas is generated from aircraft engine bleed air. Preferably, the introduction of high-purity non-combustible gas occurs during take-off and level flight of an airplane and introduction of the lower-purity gas occurs during descent of the airplane.